1. Field of the Invention
The present invention concerns a method for generation of an anatomical image of an examination area with a magnetic resonance apparatus.
The invention also concerns a computer program and a magnetic resonance apparatus for implementation of the method.
2. Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified way, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla and more) in an MR such that the nuclear spins of the examination subject orient along the basic magnetic field.
To trigger nuclear magnetic resonance signals, radio-frequency excitation pulses are radiated into the examination subject, the triggered nuclear magnetic resonance signals are measured and MR images are reconstructed based thereon. The MR imaging enables image contrasts that result from the combination of multiple parameters. Important MR parameters are, for example, the density of the excited nuclear spins (primarily hydrogen protons); the relaxation times for magnetizations (T1, T2, T2*) of the examined tissue; the magnetization transfer; and diverse additional contrast mechanisms.
For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored in a k-space matrix as complex number values. An associated MR image can be reconstructed from the k-space matrix populated with values by means of a multi-dimensional Fourier transformation.
Depending on the type of the examination and of the examination subject, an acquisition sequence is selected that exhibits those MR parameters that generate an advantageous image contrast for the examination. Value maps in which the distribution of only a single MR parameter is listed make the diagnosis easier for specific examinations.
For example, in the functional imaging of cartilage tissue value maps of the relaxation times T2 and T2* have been used for some time in order to monitor the course of a therapy or an illness (such as, for example, osteoarthritis). For this purpose, multi-echo gradient echo sequences or multi-echo spin echo sequences are used for generation of T2* maps or T2 maps, wherein the measured data of the respective multi-echo sequences are fitted to the respective relaxation equations in order to obtain a value map of the corresponding relaxation parameter.
Maier et al. describe such a procedure in “T2 Quantitation of Articular Cartilage at 1.5 T”, Journal of Magnetic Resonance Imaging 17: 358-364 (2003) using a T2 value map in connection with examinations of patellae.
Further application fields of such parameter value maps to support a diagnosis are, for example, the field of liver examinations, in particular for examination and monitoring of an iron uptake of the liver (hemachromatosis) or examination of the nerve bundle at a spinal column.
T2* and T2 value maps can also be generated in a known manner without multi-echo sequences when a number of individual measurements of an examination area are implemented in order with the same repetition time but different echo times. The generation of T1 value maps is likewise known, for example from a series of measurements (at least two) with different repetition times TR but the same echo time. Further measurement sequences are known that are used for a generation of MR parameter value maps, also with regard to other MR parameters.
In order to increase the usage of these parameter value maps and in particular to enable a precise localization of the conditions presented in the parameter value map, however, the parameter value maps should be associated with a corresponding anatomical image. However, this procedure requires a high degree of qualification and training since the parameter value maps must be manually adapted to an associated anatomical image.
Moreover, specific image information of the anatomical image can interfere with the desired information about, for example, the cartilage tissue in the combined image. For example, osseous tissue that is not important for the monitoring of cartilage tissue but possibly exhibits similar contrasts can optically deflect.